What is ‘toxic’?

When do we consider something toxic? When discussing toxic minerals, debate often arises over how dangerous something truly is. In such discussions, the 16th-century physician/alchemist Paracelsus (Theophrastus Bombastus von Hohenheim, 1493–1541) is frequently quoted:

“Sola dosis facit venenum”

Which translates to: All things are poison, and nothing is without poison; only the dose makes a thing not a poison.

There is a core of truth in that statement. A pinch of salt on your egg is harmless, but eating a cupful of salt would soon make you ill; that amount of salt would act harmfully on your body (though salt itself is not considered a poison). Indeed, even water can be deadly in excessive amounts.

However, it is worth placing Paracelsus’ words in context. He lived in the 16th century, when people had very different ideas about how the human body functioned (even though Paracelsus himself held rather unconventional medical views for his time). He made this remark in response to criticism from colleagues regarding the medicines he prepared, which contained arsenic, antimony, and his favourite ingredient, mercury. So, we should be cautious not to use this quotation to trivialise real dangers.

Essentially, the statement holds: in small quantities, most substances are not lethal and have even been used as medicines. Yet you and I generally do not know where that threshold lies, nor how much of a substance we might be exposed to in practice. Moreover, Paracelsus did not know about genetics, the importance of timing of exposure, or how chemical bonding affects a substance’s toxicity, all factors that play a significant role in determining how poisonous something is to an individual.

Returning to the question of what “toxic” means, it is not an easy one to answer.

Acute and chronic toxicity
A distinction can be made between substances that cause acute poisoning and those that are harmful only after long-term exposure. The first category is easier to measure. Toxicity is tested by exposing laboratory animals (usually mice, rats, or rabbits) to a substance and determining the amount that causes death in half of the test population. This is expressed as a value per kilogram of body weight and is known as the LD₅₀ (lethal dose 50%), the quantity required to kill 50% of the exposed animals.

A further distinction may be made between oral intake (ingestion) and dermal absorption (through the skin). Toxicity values may also be given for inhalation or dissolution in water, known as the LC₅₀ (lethal concentration 50%).

It is important to note that substances are not equally toxic in all forms. A substance may be relatively harmless in its elemental (solid, native, or crystalline) state but become highly dangerous when ground, dissolved, or chemically altered. Nor are all people affected equally: toxicity is measured per kilogram of body weight, so something harmful to a 15-kilogram child may not be equally harmful to a 90-kilogram adult. General health and individual physiology also play major roles.

The LD₅₀ value is therefore often used as a reference point when determining the level of warning required for a given substance.

Factors affecting toxicity
Various factors influence the potential toxicity of a substance, such as:

• Gender
• Body size and build
• Physical condition
• Time of exposure or ingestion
• Route of exposure
• Combination with other substances, medicines, or herbal products
• Bioavailability and solubility of the substance

The state of aggregation of a substance is also crucial: some solids are harmless, while the same substance in gaseous form can be deadly.

Chronic poisoning
Measuring the dangers of long-term exposure is much more complex. Chronic poisoning is often difficult to recognise or prove, as the link between exposure and effect is not immediately visible. Such poisoning can occur, for example, through the gradual accumulation of heavy metals in the body from repeated, low-level exposure that the body cannot process.

This is why practices such as drinking “gemstone water”, water in which minerals containing heavy metals have been placed, are strongly discouraged. In very small quantities, most substances are harmless or even naturally present in the body, but an excess can make you ill. In this, Paracelsus was indeed correct.

Bioaccumulation also occurs in other contexts, such as smoking or through the food chain. For instance, a small fish that eats algae containing traces of heavy metals is not greatly affected. However, a predatory fish that eats many such small fish accumulates these metals in its tissues. When humans then consume the predator fish, they ingest those stored metals. Chronic poisoning can lead to cancer, birth defects in children of exposed parents, and allergies.

Toxicity in minerals
When considering minerals, it is also important to look at how elements are combined. The elements within a mineral are chemically bound together, but not all bonds are equally strong. This affects how easily a harmful element can be released, for example, when the mineral is placed in water or comes into contact with acid or bodily fluids.

Thus, not all arsenic-bearing minerals are equally dangerous, as the arsenic is not always easily released. Similarly, mercury-containing cinnabar (mercury sulphide) is less hazardous than elemental mercury itself. Metal sulphides tend to form strong, poorly soluble compounds. However, if a cinnabar crystal is damaged, powdered, or crushed, caution is required. Grainy and brittle ‘massive’ forms of cinnabar are also more dangerous than well-developed crystals. Numerous cases of mercury poisoning have been recorded among miners working with cinnabar, as pure mercury can be released from crushed ore, and inhaling its vapour is extremely dangerous. Thus, there is always some degree of risk.

Safety guidelines
A good general rule is to treat all minerals containing toxic elements with caution:

• Wash your hands after handling them.
• Keep them away from children.
• Store them in sealed boxes or display cabinets.
• When opening such a container, avoid placing your face directly above it.

Always know what you have in your collection: identify your minerals accurately so you can research their composition and associated risks.